The biology of reconstitution of T cell populations following acute loss has, in the past, been incompletely characterized. Using murine models, we first identified two primary pathways of T cell immune reconstitution, the classic, thymic-dependent pathway, and a second, thymic-independent pathway. We then identified T cell surface markers which allowed identification, by phenotyping of reconstituted T cell populations, of the pathways which had given rise to them, and then applied this information to the characterization of T cell reconstitution in patients. Initial work established the applicability of this approach to the study of T cell population regeneration in humans. Recent work verified validity of the approach and established the course of T cell immune reconstitution in adult humans over time for each of the two primary pathways. This work also showed an essential role for the thymus in regenerating CD4+ T cells quantitatively. CD8+ T cells depend more strongly on peripheral expansion for immune reconstitution. Both CD4+ and CD8+ T cells require thymic activity for maintenance or generation of repertoire diversity. These findings have led in turn to a research emphasis on understanding mechanisms which control thymic function, and new treatments, including vaccine strategies, to treat cancer in the setting of a regenerating immune system. Four models of thymic regulation have been developed; this work has progressed to the developoment of two new project areas of research -- one focused on points of regulation of thymus function and one on introducing agents into clinical trials. The work addressed in this project has also led to efforts in investigating IL-7 effects on the maturation of thymocytes. We have identified IL-7 as a negative regulator of thymopoiesis as well as being essential for thymocyte development. These dual roles are dose dependent and negative regulation at higher concentrations is mediated through control of Notch signaling -- which is central to T/B lineage commitment. Additionally, we have worked to identify genes which might regulate the thymus, and have characterized a gene called SPATIAL which is a negative regulator acting within the stromal cell compartment. It appears to exert its effect through control of cell cycle.